Ingot mould with multiple angles for loaded continuous casting of metallurgical product

ABSTRACT

The invention concerns an ingot mould comprising in succession, in the direction for extracting the metallic product to be cast ( 7 ): a preheater ( 5 ) made of noncooled refractory material acting as reservoir for the melting metal to be cast and a standard cooled tubular metal element ( 6 ) for solidifying the metal. A slot ( 18 ) for injecting the shearing gas (for example Ar) is arranged between the preheater ( 5 ) and the metal clement ( 6 ) so as to emerge on the ingot mold internal periphery. The injection slot comprises means ( 17 ) for reducing the gas flow in each of the ingot mold angles, preferably formed by obstructing elements. The invention enables to reduce, even eliminate, defects encountered along the edges of the solidified cast products.

BACKGROUND OF THE INVENTION

The invention relates to a head of a mould for the hot-top continuouscasting of a metallurgical product, such as a steel bloom, billet orslab.

In the case of the continuous casting of a metallurgical product, amolten metal is poured into an upper part or head of a mould having avertical general disposition and extracted from this mould, via thebottom, is a peripherally solidified product.

The process called “hot-top continuous casting”, which in factconstitutes an improvement of the general continuous casting process, isused in such a way that the meniscus (the free surface of the castmetal) is transferred upstream of the level where the solidification ofthe metal inside the head of the mould starts. In order to carry out thehot-top continuous casting process, the usual copper tubular element ofthe mould, cooled by internal circulation of cooling water, issurmounted, perfectly contiguously, by an uncooled feed head made ofthermally insulating refractory material, serving as a reservoir ofmolten metal fed by the pouring jet from a tundish placed a shortdistance above it. By virtue of this novel type of construction of themould head, the liquid-metal meniscus is established therein, during thecasting run, within the refractory feed head, whereas the solidificationof the metal starts only level with the cooled metal tubular elementwhich, as in conventional continuous casting, calibrates the castproduct in terms of shape and size. Consequently, the stirring of theliquid metal due to the pouring jet is limited within the feed head. Inthe solidification space defined by the copper tubular element placedbelow, the flow of cast metal may thus be maintained in a relativelycalm hydrodynamic state, thereby making it possible in particular toeven out the solidification profile of the steel in contact with thecooled copper wall all around the inner perimeter of the mould. However,in order to use such a process satisfactorily, it is necessary to avoidany premature solidification of the cast metal in the feed head so as tobe able to ensure that the solidification starts lower down, preciselyat the point of contact with the cold copper wall.

To do this, it has already been proposed to leave a gap of very smallwidth (less than 1 mm and generally about 0.2 mm) between the refractoryfeed head and the copper tubular element and to inject, via this slot, afluid, generally an inert gas such as argon, into the mould around itsinner periphery. In order to ensure gas flow at any point in the slot,the latter is fed with pressurized gas via a distribution chamber whichsurrounds it.

This injection of gas has the effect of shearing the heterogeneousparasitic solidified film which could form above, against the inner wallof the refractory feed head, and thus create conditions conducive to asharp and even onset of solidification in the cooled copper elementlocated just below.

In the case of non-circular moulds, in other words in the case of mouldsprovided with a cooled tubular element quadrangular in shape (forcasting slabs, blooms or billets of square cross section, for example)or more generally multiangular in shape (for casting blanks alreadyhaving the shape of the desired end product), it has been observed, onthe cast products after complete solidification, that there aresolidification defects along the edges, such as longitudinal cracks,exfoliations, etc., defects whose origin can be identified as being alack of solidified metal at these points already in the mould, andtherefore at the very moment that the solid shell forms.

SUMMARY OF THE INVENTION

The object of the present invention is specifically to provide asolution making it possible to reduce, or even to completely eliminate,these solidification defects in the corners of the cast productsobtained.

For this purpose, the subject of the invention is a mould for thehot-top continuous casting of molten metals, comprising a cooled metaltubular element of quadrangular shape, defining the shape and size ofthe cast product and in which the molten metal solidifies on contactwith the cooled inner metal wall, the said cooled tubular element beingsurmounted by an uncooled feed head made of thermally insulatingrefractory material defining a reservoir of molten metal to besolidified, a slot for injecting a shearing fluid (especially apressurized inert gas, preferably such as argon) around the innerperiphery of the mould being provided between the cooled metal elementand the refractory feed head, the said mould being characterized in thatit is provided with means for reducing the flow of shearing fluid in thecorners.

Preferably, these means consist of an element forming an obstacle to theflow of the gas in the injection slot, the said element being placed ineach of the corners of the slot.

The invention results from the following considerations. In order toobtain a satisfactory shearing effect on the flow of gas injected at thebase of the feed head, it is necessary to maintain a gas flow rate allalong the slot so that there are no dead regions where undesirablesolidification fragments would therefore persist. However, even if theslot is fed from a peripheral pressurized-gas manifold, and thereforeensuring that head losses are equal and, consequently, that there is alinear emerging flow with a constant flow rate over the entire length ofthe slot, an injected-gas flow rate equal at every point around theperimeter of the cast product is not obtained. This is because there isa greater flow rate of gas in the corners of the mould due to the factthat, since the slot is, of course, of the same rectangular shape as themould, the inside of the latter is fed with gas in two directions in itscorner regions. This greater flow rate in the corners results, in theregion of the slot, and therefore in particular in the upper part of thecooled copper element located just below, in an overpressure which cancause local separation of the solidified shell from the cold copper wallat the edges of the cast product. It is these separations which, becauseof the collapse in the effectiveness of the product cooling in thecorners which results, are responsible for solidification-disturbingphenomena of the “lack of solidified metal” type, which phenomena arethen manifested, on the cast product obtained, by solidification defectsin the corners along the edges.

In order to make the invention more clearly understood, a descriptionwill now be given, by way of non-limiting example and with reference tothe figures appended hereto, of a mould for the hot-top continuouscasting of a steel billet of square shape according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic half-view, in axial cross section, of the upperpart of the mould on the plane 1—1 in FIG. 3.

FIG. 2 is a schematic half-view in axial cross section of the upper partof the mould on the plane 2—2 in FIG. 3.

FIG. 3 is a top view of the lower part of the mould on the plane 3—3 inFIG. 1 or in FIG. 2.

FIG. 1 and FIG. 2 show the upper part of a hot-top continuous castingmould denoted overall by the reference number 1, which has a cooledcopper tubular element 6 extended upwards, and completely contiguouslyin order to prevent any infiltration of molten metal, by a feed head 5made of uncooled refractory material.

The cooled metal element 6 and the refractory feed head 5 define, intheir internal part, an internal casting space 3 into which a moltenmetal 4, such as steel, is poured and solidifies. As may be seen in FIG.3, the internal casting space 3 has a cross section in the form of asquare with rounded corners, the radius of which has been exaggeratedlyincreased on purpose in order to more clearly show the characteristicelements making up the invention, which will be explained again below.

It will be noted that the cooled copper tubular element 6 forms the mainelement of the mould. It is this element which, being vigorously cooledby an internal circulation of water (which takes place here in a space 2left between the element 6 and a metal jacket 8 which surrounds thelatter at some distance therefrom), conventionally serves as acrystallizer, against the inner wall 11 of which the molten steel 7solidifies, forming firstly a first shell 7′ as soon as the steel firstcomes into contact with the cold copper 11. Next, as the cast productprogresses downwards in the mould in the direction indicated by thearrow F, this shell, under the effect of the intense heat pumping due tothe vigorous cooling of the copper element 6, steadily thickens. Thesolidification of the cast product 7 thus progresses from the peripherytowards the central axis until complete solidification, whichconventionally occurs about ten metres below the mould, water spraysbeing provided for this purpose following the mould in order toimmediately spray the surface of the cast product to be cooled.

As regards the feed head 5, which is a specific component of so-called“hot-top” casting, its essential function is to serve as a reservoir 4of molten metal. This metal arrives as a pouring jet 12 coming from atundish 14, placed a short distance above it, via a nozzle 13 mounted onthe outlet orifice of the tundish. The reservoir 4 constitutes a buffermass, which plays a key role with regard to the hydrodynamics byallowing the often violent stirring of liquid metal due to the greatmomentum of the steel jet 12 to freely develop therein and therefore tobe damped therein. Thus, the liquid steel which then enters thecrystallizer 6, in order to solidify in it, is in a much calmer stateand, above all, far from the meniscus 15, the stirring of which is oftenthe cause of solidification heterogeneities in the outermost shell in aconventional continuous casting mould. Beneath the reservoir 4, the flowof the molten metal approaches “piston”-type flow, that is to say flowwithout a marked gradient in the velocity vector across the section,something which is extremely favourable to the proper execution of thesolidification process.

As a general rule, but not shown in the figures, the feed head 5 made ofrefractory material has a main upper part made of a fibrous refractorymaterial chosen for its thermal insulation properties so as to keep thereservoir of molten metal 4 in the liquid state, for example thematerial sold under the name A120K by the company KAPYROK, and a lowerannular insert chosen to be made of a dense refractory material, such asSiAlON® in order to ensure the best mechanical integrity in theimmediate vicinity of the cooled copper element 6 stressed by the onsetof solidification.

It will be seen that the feed head is fastened, in a position wellaligned with respect to the tubular element 6, by means of alignmentpins, not shown, and of an assembling flange 9 with a tie rod 9′, thisflange bearing on a metal plate 5 a covering the refractory part. A box10 made of sheet metal is advantageously provided for the passage of thetie rods and in order to stiffen the assembly.

Despite the thermal insulation properties of the refractory materialused for the feed head 5, parasitic solidified films 16 of cast metal,of greater or lesser extent, may form on the inner wall of the feedhead. Even localized on the perimeter, they can be deleterious tocorrect solidification in the crystallizer 6 in so far as thesefragments 16 may reach as far as level with the edge of the cooledelement 6 where the solidification starts. In order to break, beforethis stage, any undesirable solidified film formed prematurely in thefeed head, a shearing fluid is injected peripherally at the base of thefeed head. In this regard, it would be preferable to use a gas, and evenmore preferably a gas which is chemically inert with respect to the castmetal, such as argon.

To this end, a narrow slot 18, for example with a width of about 0.2 mm,is provided between the feed head 5 and the cooled copper element 6.This slot opens freely towards the inside of the mould and emerges atits other end in a sealed annular chamber 19 provided in the feed head.This chamber 19, which runs all along the slot 18, serves to properlydistribute the linear flow of gas that has to emerge from the slot. Itis connected via a duct 20 to an external source 21 of pressurized gas.The slot 18 has an annular shape similar to the quadrangular shape ofthe mould, and therefore to that which the cast product 7 adopts oncethe shell has solidified within the copper element 6. In particular, ittherefore has an outline with four corners, as shown in FIG. 3, wherethe rounded part of the corners has been deliberately exaggerated forthe reasons mentioned above.

Because near each of the corners 3 a, 3 b, 3 c and 3 d of the mould theshearing gas introduced into the casting space 3 is supplied from twosides of the slot 18 at right angles, the two-directional and convergentfeed in the corner regions of the casting space 3 means that more gas isblown into these regions, entailing a risk of localized separation ofthe cast metal from the copper wall 11 at the upper edge of the latter,at the point where the outermost shell forms, and, consequently, meansthat there is insufficient solidified metal, compared with the rest ofthe perimeter, in the region of the edges of the cast product duringsolidification within the copper element 6, because of the lack ofeffective cooling of the product at these points.

In order to prevent this excess injection of gas into the cornerregions, elements for obstructing the flow of the gas are placed,according to the invention, in the corners of the slot 18, as may beseen in FIGS. 2 and 3.

The obstructing elements 17, placed in corners of the gap 18, mayconsist of bundles of flexible fibrous refractory material which, afterthe feed head has been clamped against the top of the metal element 6,locally block the passage, by flattening, from the outside towards theinside of the mould. Each of the obstructing elements 17 is thenadvantageously bounded towards the outside by the internal perimeter ofthe distribution chamber 19, towards the inside by a corner of thecasting space 3, and laterally by two straight sides converging towardsthe casting space 3 and making an angle α with the perpendicular to theplane internal surface of the casting space 3, at the corresponding endof the rounded corner 3 a (or 3 b, 3 c, 3 d, respectively) of thecasting space which delimits, inwardly, the obstructing element 17.

If the rounded corner of the casting space of the mould has a radius ofabout 6.5 mm, the width of the obstructing element 17 in its narrowestregion, adjacent to a corner of the casting space, must preferably bebetween 4 and 6.5 mm. If this width is less than 4 mm, the localizedexcess flow of gas injected into the corner is not properly eliminated.If the width is greater than 6.5 mm, there is a region near the cornerwhere there is no linear flow of injected gas.

Moreover, the angle a between the straight side of the obstructingelement 17 and the perpendicular to the internal surface of the castingspace will advantageously be between 0 and 45°. Outside these values ofthe inclination of the sides of the obstructing element 17, the linearflow of injected gas, that is to say the flow per unit length of theinner perimeter of the mould level with the slot 18, becomes zero in aregion near the corners.

It has been found that a value of the angle α of about 20° makes itpossible to obtain a constant linear flow around the inner perimeter ofthe mould in the case of the casting of products of rectangular orsquare shape. In certain cases, depending on whether the shape of theproducts to be cast is more or less complex, the two straight lateralsides of the obstructing elements 17 may make different angles α and α′with the perpendiculars to the plane internal surface of the internalcasting space 3 at the ends of the corners.

By using elements for obstructing the slot 18 which have the geometricaland dimensional characteristics given above, it is possible to obtain alinear flow of inert gas into the internal casting space, at the slot18, which is perfectly constant. In this way, the solidification defectsobserved along the edges of the cast product once it has solidified areeliminated.

The invention is not limited to the embodiment which has been described.For example, it is possible to use, as the element obstructing the slot18 in its corner regions, materials different from refractory fibres.These elements may be completely impermeable to the gas, or elseslightly porous.

It is also possible to obstruct the slot 18 in its corner regions and toeliminate the gas flow in these regions by making the feed head 5slightly thicker in the corner regions extending over the width of theslot 18, between the internal casting space 3 and the distributionchamber 19. This additional thickness may be achieved by machining, forexample by milling, the lower face of the feed head 5 adjacent to theelement 6. Conversely, the additional thickness in the corner may beobtained on the element 6, that upper face of which, facing the feedhead 5, would be machined for this purpose. Preferably, the region ofadditional thickness will have a shape similar to the shape of theobstructing elements 17 as illustrated in FIG. 3. This additionalthickness may be preferably about 0.2 mm.

It is also possible to partially obstruct the distribution chamber 19 inregions close to its corners, so as to limit or to eliminate theinjection into the corner regions of the slot 18. The distributionchamber may be obstructed, for example, by introducing, into the cornerregions of the distribution chamber, plugs penetrated by channels in thedirection of flow of the gas in the distribution chamber or else plugshaving a degree of porosity.

The invention applies to any multiangular mould head for the hot-topcontinuous casting of a metallurgical product, such as a billet, a bloomor a slab, or blanks of a shape already close to the end product,(beams, rails, various sections, etc.) provided that the head satisfiesits definition given by the appended claims. Moreover, it may be appliedboth in the case of the continuous casting of steel and in the case ofthe continuous casting of non-ferrous metals.

What is claimed is:
 1. Mould for the hot-top continuous casting ofmolten metals, comprising a cooled metal tubular element (6) ofmultiangular shape, defining the shape and the size of a cast productand in which the molten metal (7) solidifies on contact with a cooledinner metal wall (11), said cooled tubular element being surmounted byan uncooled feed head (5) made of thermally insulating refractorymaterial defining a reservoir of molten metal to be solidified, a slot(18) for injecting a shearing fluid around the inner periphery of saidmould being provided between said cooled metal element (6) and saidrefractory feed head (5), wherein said mould comprises means (17) forreducing the flow of shearing fluid in a pluarality of corners locatedin said slot.
 2. Mould according to claim 1, characterized in that themeans for reducing the gas flow consist of elements (17) for locallyobstructing the flow in the slot (18).
 3. Mould according to claim 2,characterized in that the obstructing elements (17) each consist of abundle of compressed fibrous refractory between the feed head (5) andthe cooled tubular element (6) and each is placed in a corner region (3a . . . 3 d) of the slot (18).
 4. Mould according to claim 2,characterized in that the elements (17) for obstructing each of saidcorner regions of the slot (18) have two straight lateral sides betweena distribution chamber (19) and a corner (3 a, 3 b, 3 c, 3 d) of aninternal surface of an internal casting space (3) converging towards thecasting (3) and each making an angle of between 0° and 45° with aperpendicular to the internal casting surface (3) near a corner (3 a, 3b, 3 c, 3 d) of the surface of the internal casting space (3).
 5. Mouldaccording to claim 2, and having rounded corners with a radius of about6.5 mm, characterized in that the obstructing elements (17) have a face,which faces the casting space (3), with a width of between 4 and 6.5 mm.6. Mould according to claim 1, characterized in that the means forreducing the gas flow consist of elements for partially obstructing thecorners of the injection slot (18).
 7. Mould according to claim 3,characterized in that the elements (17) for obstructing each of saidcorner regions of the slot (18) have two straight lateral sides betweena distribution chamber (19) and a corner (3 a, 3 b, 3 c, 3 d) of aninternal surface of an internal casting space (3) converging towards thecasting (3) and each making an angle of between 0° and 45° with aperpendicular to the internal casting surface (3) near a corner (3 a, 3b, 3 c, 3 d) of the surface of the internal casting space (3).
 8. Mouldaccording to claim 3, and having rounded corners with a radius of about6.5 mm, characterized in that the obstructing elements (17) have a face,which faces the casting space (3), with a width of between 4 and 6.5 mm.9. Mould according to claim 4, and having rounded corners with a radiusof about 6.5 mm, characterized in that the obstructing elements (17)have a face, which faces the casting space (3), with a width of between4 and 6.5 mm.
 10. Mould according to claim 7, and having rounded cornerswith a radius of about 6.5 mm, characterized in that the obstructingelements (17) have a face, which faces the casting space (3), with awidth of between 4 and 6.5 mm.